Photo-active member for xerography



Feb. 11, 1964 Filed June 26, 1957 A. E. MIDDLETON ETAL PHOTO-ACTIVE MEMBER FOR XEROGRAPHY 6 Sheets-Sheet 2 ATTORNEY Feb. 11, 1964 A. E. MIDDLETON ETAL 3,121,006

PHOTO-ACTIVE MEMBER FOR XEROGRAPHY 6 Sheets-Sheet 3 Filed June 26, 1957 0; 32086 626520 EE M2; 09 ow ow ow om o o 280 E0 J Tkuwo x59 om mmomi ow 89 f ow p on 1 oo. mmmm lllllll/ oom oov SL'IOA IVILNBLOd 3.1.V'lcl HAILV'DHN m O mozoumm 626N210 E52 M2; om ow 0 INVENTOR.

Arthur E. Middleion Donald G. Reynolds @OMN n CON- SL'IOA "IVLLNBLOd 3.1.V'ld EALUSOd ATTORN Feb. 11, 1964 Filed June 26, 1957 DLI A. E. MIDDLETON ETAL PHOTO-ACTIVE MEMBER FOR XEROGRAPHY 6 Sheets-Sheet 4 I o Phosphor F- 2039-Zn0 A Phgsphor I 225 -ZnO 5 l V Phosphor 2330-Zn0 D ZnO 4 X 3 \q 2 Wavelenqih m FIG. 7

INVENTOR. Arthur E.M|dd|eton BY Donald C. Reynolds ATTORNEY 1964 A. E. MIDDLETON ETAL 3,

PHOTO-ACTIVE MEMBER FOR XEROGRAPHY 6 Sheets-Sheet 5 m OE ril- N QN ON 0m Na MN Filed June 26, 1957 Arthur E. Middleton Don Id C. Reynol 5 ATTORNEY 1964 A. E. MIDDLETON ETAL 3,

PHOTO-ACTIVE MEMBER FOR XEROGRAPHY 6 Sheets-Sheet 6 Filed June 26, 1957 0 O o o O. 7 6 5. 4 3 ac: 5.33.6 33cm Wavelenglh, rn,u

FIG. IO

INVENIOR. Arthur E. Mlddleton Donald C. Reynolds ATTOR EY United States Patent PHQTQ-ACTIVE Mllitllfiljll FGR XERE GRAPHY Arthur E. Middleton, Indianapolis, ind, and Donald C.

Reynolds, Springfield, Ohio, assignors, by mesne assignments, to Xerox Corporation, a corporation of New York Filed June 26, 1957, Ser. No. 663,165 15 Claims. (Cl. 96--1) This invention relates in general to xerography and in particular to xerographic plates and a Xerographic process using such plates. More specifically, the invention relates to a new Xerographic member comprising a rel.- tively conductive backing having on at least one surface thereof a coating of a finely ground photoconductive insulating material dispersed in a high resistance electrical binder.

in the xerographic process as described in US. 2,297,- 691 to C. F. Carlson, a base plate of relatively low electrical resistance such as metal, paper, etc. having a photoconductive insulating surface thereon is electrostatically charged in the dark. The charged coating is then exposed to a light image. The charges leak otf rapidly to the base plate in proportion to the intensity of light to which any given area is exposed. After such exposure the coating is contacted with electrostatic marking particles in the dark. These particles adhere to the areas Where the electrostatic charges remain forming a powder image corresponding to the electrostatic image. The powder image can then be transferred to a sheet of transfer material resulting in a positive or negative print, as the case may be, having excellent detail and quality. Alternatively, where the base plate is relatively inexpensive, as of paper, it may be desirable to fix the powder image directly to the plate itself.

As disclosed in Carlson, suitable photoconductive insulating coatings comprise anthracene, sulfur r various mixtures of these materials as sulfur with selenium, etc. to thereby form uniform vitreous coatings on the base material. These materials have a sensitivity largely limited to the blue or near ultraviolet and have a further limitation of being only slightly light sensitive. Consequently, there has been an urgent need for improved photoconductive insulating materials. The discovery of the photoconductive insulating properties of highly puii fied vitreous selenium has resulted in this material becoming the standard in commercial Xerography.

The photographic speed of this material is many times that of the prior art 1 hotoccnductive insulating materials. However, vitreous selenium suffers from two serious defecis: first, its spectral response is very largely limited to the bllic or near ultraviolet; and, second, the preparation of uniform films of vitreous selenium has required highly involved and critical processes, particularly vacuum evaporation. Furthermore, vitreous selenium by its nature requires a relatively firm and uniform support such as a continuous plastic or mteal base. This, together with the high cost of selenium itself has rendered impractical the development of a disposable xerographic plate such as a paper base plate using this material.

Now, in accordance with this invention, it has been found that a Xerographic sensitive member known as a xerographic plate can be prepared by intimately mixing and grinding together a photocon uctive insulating material in a high electrical resistance bi .der. This mixture is suitable as the photoconductive insulating layer in the xerographic plate and may be coated on any suitable support material offering a relatively lower electrical resistance such as metal, paper, suitable plastics or conductively coated glass, plastics, etc. as more fully dc scribed hereafter.

This composition completely obviates the necessity for BJZLdhS Patented Feb. ll, 19W

such procedures as vacuum evaporation. One of the advantages of the novel Xerographic plates prepared according to the instant invention is the ease and. variety of means or" applying the photoconductive insulating layers to a base material. The photoconductive insulating layer itself comprises an inor anic photoconductive: insulating compound dispersed in a high electrical resistance binder. This composition dissolved in a suitable solvent may be flowed on the base material or otherwise coated on the base as by dipping, whirling, the use of a doctor blade, dip roll etc. Alternatively, the composition may be rendered flowable using a thermoplastic resin as the insulating binder and heat to render the composition plastic. In this form the composition may be applied to the base material without the necessity for a solvent. Yet again, a solvent solution of the coating composition may be emulsified or dispersed in water and the aqueous emulsion or dispersion coated on the base material.

Plates so prepared possess a number of unequaled and useful advantages. Thus, the ease and variety of means of applying the coatings together with the large degree of flexibility inherently possessed by such pigmentbinder compositions renders such photoconductive insulating layers eminently suitable for application to a variety of substrates and, in particular, they are easily applied to such inexpensive substrates as paper and similar felted fibrous bases.

There are disclosed in U. S. Patent application Ser. No. 95,374, filed on May 25, 1949, by Arthur E. Middleton, now US. Patent 2,663,636, various methods and means whereby any photoconductive insulating material in an insulating resin binder can be formed into an operable xerographic plate. There are both photoconductive insulators and photoconductive semi-conductors. Both materials show a resistance considerably lower in the light than in the dark. Both materials would be insulators at a temperature of absolute zero. The difference between these photoconductors lies in their ability to hold charges in the dark. A photoconductive semi-conductor will not and for the purposes of xerography such a material is not useful. Hence, these materials are generally restricted to photccells, rectifier-s and similar applications.

On the other hand photoconductive insulators will hold a charge in the dark and can, thus, be used in the xerographlc process. Where the photoconductor forms a homogeneous layer, its ability to hold a charge is essentially cle endent on the dark resistivity of the photoconductive insulator itself. When used in this manner the minimum operable dark resistivity is generally in the order of 10 ohmscrn. and it is preferred that the material be more resistant. Very few photo conducting materials possess such a high resistivity in the dark. When the photoconducting material is incorporated in an insulating binder, a larger part of the resistivity of the component layer is dependent on the resistivity of the resin binder. Accordingly, the darlr resistivity requirements for the photocouductive insulator are not nearly so strenuous. In general, a material is considered a photoconductive insulator for use in a binder plate if it shows a resistivity in the dark above about 10 ohmscm. it is evident that all insulators and all semi-conductors are not photoconductors. Certain arrangements among the allowed electron energies in the material are required to achieve photoconductivity. Thus, technically, photcconductive insualting is a defining term distinguishing the material from an insulator, a semi-conductor and a photoconductive semi-conductor.

Resistivity is a simple physical property which may be determined by consulting an appropriate handbook or by a simple electrical measurement. Photocond'uctivity as used herein is more illusive of accurate definition. In general, photoconductivity requires making electron transitions to the conduction band upon the absorption of light. There are certain definite physical properties generally associated with materials possessing this ability. While not all members of each class are necessarily photoconductive insulating compounds as described herein and hence, operable in a xerographic binder plate, nevertheless, the physical properties constituting the distinguish ing characteristics of the group also constitute extrinsic evidence of photoconductivity. Hence, the photoconductive members of each group possessing the requisite resistivity as herein defined are photoconductive insulating compounds. Thus, the inorganic photoactive compounds operable in the instant invention may be classified in these groups: first, inorganic luminescent or phosphorescent compounds; second, inorganic, intrinsically colored compounds having an index of refraction of at least 2; third, inorganic compounds possessing at least one index of refraction greater than 2.10 over at least 5% of the wavelength range of visible light; and, fourth, inorganic compounds which have two different valence states of at least one elemental constituent between which electron transfers can occur.

First, in general, luminescence or phosphorescence is evidence of the elevation of one or more electrons to a higher energy level. Therefore, whenever the elevation is sufficient so that the electric charge is free to migrate upon the application of an electric field the material is photoconductive. It is assumed that materials classed herein as phosphors are either intrinsically phosphors or are made so by the inclusion or addition of specified impurities or promoters as is well known to those skilled in the art.

Another class of inorganic photoconducting insulating compounds is intrinsically colored ionic compounds. Colored compounds, due to their greater light absorption, have a greater light eliiciency than colorless compounds at certain wavelengths. An intrinsic color is the term used to describe the color resulting from the interaction of ions on each other whereby there is produced a color different than that produced by the ions separately and additively. The interaction is a property of the compound itself and is a constant phenomenon, that is, it is the same by whatever process the compound is prepared. Thus, an intrinsic color is direct physical evidence either of a maintained distribution of excited valence orbitals or of the presence of constantly occurring electron shifts. Such materials are often photoconductors. This prop erty by itself is not, however, sufiicient evidence of photoconductivity. Another physical property related to photoconductivity is the refractive index. The refractive index is equated to the square root of the dielectric constant and is directly proportional to the number of mobile electrons, that is, electrons free to vibrate. Such mobile electrons are easily excited to the conductive band by the absorption of energy which is the necessary requirement for photoconductivity. The index of refraction gives a rough approximation of the number of such electrons. When the index of refraction is at least two and we have the concurring evidence of intrinsic color, the compound is generally photoconductive. Thus, another class of inorganic compounds are photoconductive insulating compounds which are intrinsically colored and possess an index of refraction of at least 2.

Still another class of photoconductive insulating compounds are those inorganic compounds which have at least one index of refraction greater than 2.10 over at least 5% of the wavelength range of visible light. For this class of compounds it may be considered that there are sufiicient free electrons that it is not necessary to have the confirming evidence of intrinsic color to classify the compound a priori as a photoconductor.

A final class of compounds in which the requsite electron transfers can occur for photoconductivity are inorganic compounds which have two different valence states of at least one elemental constituent between which electron transfers can occur. One evidence of such compounds occurs when the stoichiometric composition of the material implies the presence of an unusual oxidation state of one or more of the chemical elements present. The Berthollides are in this group, particularly that class of Berthollides called perovskites.

Generally, the inorganic photoconductive insulating compounds found suitable for use in xerographic binder plates may be described as being characterized by having electrons in the non-conductive energy level activatable by illumination to a different energy level whereby an electric charge is free to migrate under an applied electric field in the order of at least 10 volts per centimeter, the composite resistivity of the binder and photoactive material in the layer being at least 10 ohms-cm. in the absence of illumination and the decay factor being less than 3.0. The measured apparent specific resistivity of the composite layer tends to vary with the field strength and, therefore, should be determined under the approximate conditions of use which may, in the absence of other indications, be a field strength in the order of 10 to 10 volts per centimeter.

The decay factor is determined by the rate of charge decay in the absence of activating radiation. A xerographic plate, in general, is closely analogous to a condenser. The potential decay of a condenser may be expressed by the equation:

V V t/RC where:

V=potential in volts at time t V =initial potential t=time in seconds R=resistance in ohms C=capacitance in farads c base of natural logarithms In this equation, l/RC is the decay factor; when 1 /RC is large, the decay will be rapid and when it is small the decay will be slow.

Actually, for these new members, l/RC may change, as decay proceeds, but an average value of l/RC over the range employed adequately describes this critical factor. By mathematical derivation, it can be seen that if a potential decays from 200 v. to v. in as much as 100 sec., the decay factor is less than 0.01, this representing a high quality performance. Similarly, if such'decay requires only 10 sec., the factor is less than 0.1, this representing a preferred upper limit of the decay factor. As an approximate maximum, if the decay factor is about 3.0, the potential decay would be from about 200 volts to about 100 volts in about 4 sec., and this represents substantially the maximum decay rate that can be tolerated in accordance with presently known xerographic techmques.

It is evident that the gap between the valence and the conducting band of a compound is determinant of the energy needed to make electron transitions. The more energy needed the higher the frequency to which the photoconductor will respond. As have been described above, there are various external evidences of stress on the electronic configuration of inorganic compounds which are evidence of the desired electronic structure. It is obvious that it is possible to reduce the band-gap for these compounds by adding a foreign compound as an activator which either by virtue of its atomic dimensions or by possessing a particular electronic forbidden zone structure or through the presence of traps as donor levels in the intermediate zone between the valence and the conduction band stresses the electronic configuration of the photoconductive compound so as to reduce its band-gap and, hence, increased its ability to release electrons to its conduction band. Phosphors almost necessarily imply the presence of such activating substances. The effect of such impurities may be such as to confer photoconducan tivity upon a compound which intrinsically is non-photoconductive. The (Ca-Sr)S phosphors used herein are believed to be in this group. However, excessive impurity content can be deleterious. Thus, many CO1-- pounds which are intrinsically operable as photoconductive insulators are reported in the literature as photo conductive semiconductors ecause of the unsuspeceted presence of conducting impurities.

The last class of compounds described above, that is, those possessing two different valence states of the same elemental constitutent, is also thought to represent such a mechanism. These compounds include the class of compounds known as defect compounds in that they possess defects in their crystal lattices which stress the electronic structure.

It is possible for a compound to belong in more than one of these classes, sometimes depending on the method of preparation. Thus, when suitably activated as with manganese, calcium, cadmium, copper, etc. zinc sulfide is a phosphor and belongs in that group. With proper treatment, ZnS may also be classified 'as a defect compound (the fourth group in the above classification). However, this is largely a theoretical classification due to the great difficulty of not activating ZnS as to render it phosphorescent. Another example is Pb O which in the above classification belongs in both group two and group tour.

"In considering specific compounds, oxygen generally forms uncolored compounds with colorless cations. Hence, unless the oxide possesses phosphorescence or a high refractive index, as in the case of titanium dioxide, the oxides are not apt to be photoconductive. On the other hand, the remaining chalkogenides often .formintrinsically colored compounds particularly with polyvalent metals. Thus, the sulfides, selcnides and tellurides of these metals are generally photoconductive compounds.

iodides are often intrinsically colored. Mercuric iodide is an excellent photoconductor. However, the iodide ion does not always confer a high color to salts. Certain iodides, simple and complex, which contain, for example mercury, may be viewed as intrinsically colored. In the instances of Cullgl Cu ligl among others, the intrinsically is quite obvious because such compounds undergo a reversible color change from red to yellow at about 150 C. depending on the compound. Stibnides, arsenides, phosphides and similar compounds also are often photoconductive but possess complicated net strucures. The same is true of the Berthollides and perovslrites. Zinc titanate may possess the perovskite structure. Specific photoconductive insulating compounds investigated by us include but are not limited to phosphors such as zinc oxide, zinc sulfide, zinc-cadmium sulfide, zinc-magnesiiun oxide, cadmium selenide, zinc silicate, CQlCllllTleSilOIlllUl'll sulfide, etc.; intrinsically colored compounds such as cadmium sulfide, mercuric iodide, mercuric oxide, mercuric sulfide, indium trisulfide, gallium triselcnide, arsenic disulfide (A5 8 arsenic trisullide, arsenic triselenide, antimony trisulfide, red lead (1 13 0,), etc.; compounds having a high index of retraction such as titanium dioxide; and defect compounds such as zinc titanate, red lead, zinc sulfide (GP. grade) etc.

The binder material which is employed in cooperation with the photoactive compound is a material which is an insulator to the extent that an electrostatic charge placed on the layer is not conducted by the binder at a rate to prevent the formation and retention of an electrostatic latent image or charge thereon. The binder material is adhered tightly to the base material and provides efiicient dispersing medium for the photoactive particles. Further, the binder should not react chemically with the photoactive compound.

Satisfactory binder materials for the practice of the invention are polystyrene; silicone resins such as DC-SOl, DC-804, and DC996 all manufactured by the Dow Corning Corp. and SR-82 manufactured by the General Electric Company; acrylic and mcthacrylic ester polymers such as Acryloid A10 and Acryloid E72, polymerized ester derivatives of acrylic and alpha acrylic acids both supplied by Rohm and Haas Company, and Lucite 44, Lucite 45 and Lucite 46 polymerized butyl methacryiates supplied by the E. I. du Pont de Nemours & Company; chlorinated rubber such as Parlon supplied by the Hercules Fowder Company; vinyl polymers and copolymers such as polyvinyl chloride, polyvinyl acetate, etc. including Vinylite VYHH and VMCH n1anufactured by the Bakelite Corporation; cellulose esters and ethers such as ethyl cellulose, nitrocellulose, etc.; alkyrd resins such as Glyptal 2469 manufactured by the General Electric (10.; etc. In addition, mixture of such resins with each other or with plasticizers so as to improve adhesion, flexibility, blocking, etc. of the coatings may be used. Thus, Rezyl 869 (a linseed oil-glycerol alkyd manufactured by American Cyana-mid Company) may be added to chlorinated rubber to improve its adhesion and flexibility. Similarly, Vinylites VYHH and VMCH (polyvinyl chloride-acetate copolymens manufactured by the Bakelite Company) may be blended together. Plasticizers include phthalates, phosphates, adipates, etc. such as tricresyl phosphate, dioctyl phthalate, etc. as is Well known to those skilled in the plastics art.

While the nature of the resin is not critical it does have a definite effect upon the light sensitivity of the composite layer. In general, those binders having strongly polar groups such as carboxyl, chloride, etc. are preferred over the straight hydrocarbon binders. it is believed that injection of carriers from the photoconductor to the binder is facilitated through the presence of such groupings and further that the bonding of the photoactive compounds to the binder is improved thereby.

T he method of preparation of the binder has a significant effect upon its conductivity and, therefore, its operability in a xerographic plate. Certain methods of polymerization lead to the inclusion of significant quantities of ionic materials such as emulsifying agents, salts, etc. in the binder which contaminants would render inoperable a resin in itself quite operable. Furthermore, a resin may be operative with one pigment and not with another. Thus, a particular resin may have a border-line resistivity so that when blended with a high resistant p'hotoactive compound such as the proper rform of cadimiuni sulfide, there results an operable xerographic binder plate whereas when blended with a less resistive photoactive compound such as zinc oxide, the cumulative effect of the compound and binder is to result in an inoperable xerographic binder plate, i.e., one which is unable to hold an electrostatic charge in the dark.

The matter of possible reactivity between the binder and compound has been mentioned. Silicone resins are particularly subject to this disability. Thus, mercuric salts and lead salts are very apt to react with silicone resins when dispersed therein. Within these general considerations, which are obvious to any chemist skilled in the art, any material which is an insulator to the extent defined above is operative as the binder in the instant invention.

The function of the base or backing material used in preparing xerographic binder plates is to provide physical support for the photoconductive insulating layer and to act as a ground thereby permitting the photoconductive insulating layer to receive an electrostatic charge in the dark and permitting the charges to migrate when exposed to light. lt is evident that a wide variety of materials may be used, for example, metal surfaces such as aluminum, brass, stainless steel, copper, nickel, zinc, etc.; conductively coated glass as tinor indium-oxide coated glass, aluminum coated glass, etc.; similar coatings on plastic substrates; or paper rendered conductive by the inclusion of a suitable chemical therein or through conditioning in a humid atmosphere to insure the presence therein of sufficient water content to render the ma- 6 terial conductive. T act as a ground plane as described herein, the backing material may have a surprisingly high resistivity such as 16 or ohms-cm.

Where the composite layer of binder and photoactive compound has suucient strength to form a self-supporting layer (termed pellicle), it is possible to eliminate a physical base or support member and substitute therefor any of the various arrangements well known in the art in place of the ground plane previously supplied by the base layer. A ground plane, in effect, provides a source of mobile charges of both polarities. The deposition on the other side of the photoconductive insulating layer (from the ground plane) of sensitizing charges of the desired polarity causes those charges in the ground plane of opposite polarity to migrate to the interface at the photoconductive insulating layer. Without this the capacity of the insulating layer by itself would be such that it could not accept enough charge to sensitize the layer to a xerographically useful potential. it is the electrostatic field between the deposited charges on one side of the photoconductive layer and the induced charges (from the ground plane) on the other side that stresses the layer so that when an electron is excited to the conduction band by a photon thereby creating a hole-electron pair, the charges migrate under the influence of this field thereby creating the latent electrostatic image. It is thus obvious that if the physical ground plane is omitted a substitute therefor may be provided by depositing on opposite sides of the photoconductive insulating pellicle simultaneously electrostatic charges of opposite polarity. Thus, if positive electrostatic charges are placed on one side of the pellicle as by corona charging as described in US. 2,777,957 to L. E. Walkup, the simultaneous deposition of negative charges on the other side of the pellicle also by corona charging Will create an induced, that, is a virtual, ground plane within the body of the pellicle just as if the charges of opposite polarity had been supplied to the interface by being induced from an actual ground plane. Such an artificial ground plane permits the acceptance of a usable sensitizing charge and at the same time permits migration of the charges under the applied field when exposed to activating radiation. As used hereafter in the specification and claims, the term conductive base includes both a physical base and an artiiicia one as described herein.

1 he physical shape or conformation of the xerographic binder plate may be in any form whatsoever as desired by the formulator such as flat, spherical, cylindrical, etc. The plate may be flexible or rigid.

Due to the great variety of photoconductive materials used in preparing the photoconductive insulating layers in the instant invention, it is possible to prepare xcrographic plates having a variety of colors and hues with light sensitivities ranging from complete panchromaticity over the visible spectrum to sensitivity to a specific narrow range of Wavelengths anywhere from the near ultraviolet to the near infrared. A series of pigments, each in itself being sensitive to a series of wavelengths, may be combined in the photoconductive insulating layer. In cases of such combination the sensitivity in the resulting plate is not necessarily the sum of the sensitivities of the individual pigments. In some cases it has been observed that the pigment will have a quenching effect on the other pigment or pigments. The reason for this effect is not known.

.The spectral sensitivity of plates prepared in accordance with the instant invention may, as is obvious to those skilled in the art, be modified through the inclusion of photosensitizing dyes therein. The dyes useful for this purpose are those commonly used in photographic sensitization and the basic mechanism of dye sensitization in xerographic binder plates is believed to be the same as that in photographic sensitization. By using such dyes singly or in combination, it is possible to further modify and, in effect, tailor-make the resulting binder plate.

The general nature of the invention having been set forth, the following examples are now presented as illustrations but not limitations of the methods and means of carrying out the invention.

Example 1 A mixed cadmiurnsultfide zinc-sulfide phosphor commercially available under the name Phosphor 2225 (New Jersey Zinc Company) was mixed with an adhesive binder which was a silicone resin commercially available under the name DC-996 (Dow Corning Corp). The phosphor material had a particle size of about one micron. The phosphor crystals and the binder material were mixed together in equal parts by weight together with toluene in an amount equal to the volume of the binder material. The resulting mixture was painted on the surface of a mirror-finished aluminum plate and was allowed to dry. I

The product is a xerographic plate comprising a metallic backing and a photoconductive insulating layer thereon. In the absence of illumination, the photoconductive insulating surface is characterized by the ability to accept and retain an electrostatic charge of at least about 300 volts and upon suitable exposure to activating radiation rapidly to dissipate this charge. Once the cadmium-sulfide zinc-sulfide material has been exposed to radiation it has a residual electrical conductivity which causes it to dissipate an electric charge imposed on its surface, this conductivity decreasing with time and capable of being terminated by a heat or infrared quench. For this reason, the usual process steps of electrophotography must be somewhat modified when employing the sensitive member of this invention.

Thus, it is usual to carry out the xerographic process by applying an electric charge to the photoconductive insulating surface, exposing the surface to a light pattern to selectively dissipate the electric charge and thereafter developing the electrostatic latent image by exposure to an electrostatically attractable material. In the case of the present invention, it is necessary to interpose the step of quenching in order to destroy residual conductivity from a prior exposure and it is, therefore, recommended that such a quench be applied, for example, immediately prior to the charging step if the plate is to be used in a repetitive fashion. Storage in the dark will also generally restore a plate to its original sensitive condition. While the mechanism of fatigue is not understood, it is believed to be due to charge carriers being caught in traps in the photoconductor. Thermal excitation as by heating supplies the energy needed to free the carrier from the trap. Other means of relieving fatigue known to those skilled in xcrography may also be used. Where the xerographic plate is not to be reused, fatigue is not a problem, though in the case of easily fatigued photoconductors it may be desirable to store the plate in the dark until use.

Examples 2 Through 18 A series of xerographic plates were prepared in the following manner: In each case a 4 x 5-inch sheet of polished aluminum was dipped in a solution of 5% sodium hydroxide long enough to dull the finish of the aluminum. A coating emulsion was formulated by adding 10 millimeters of distilled water to 15 millimeters of an aqueous polystyrene emulsion commercially available under the name BKS-92 from the Bakelite Company, a division of Union Carbide and Carbon Company. Enough of the photoconductive material was then added to the emulsion to bring the total volume to 30 millimeters. The emulsion was then agitated and after thorough mixing, 8 millimeters was spread evenly over the metal plate. The plate was air dried, baked for 15 minutes in an oven at 250 F. and then cooled to room temperature. The plates so prepared were tested by being passed under a corona charging unit which unit had been adjusted to give a plate-to ground current of 7 microamperes when using electrically positive sensitization and 26 microamperes when using negative sensitization. Immediately after charging the potential on the plate was measured with a vibrating probe electrometer. After 30 seconds in the dark, potential was remeasured. The plate was then recharged, exposed to the light from a Bausch & Lomb photomicrograph lamp for 30 seconds and the voltage on the plate remeasured. When prepared and tested in this manner the following materials were found to accept an electrostatic charge and dissipate the charge upon illumination: Zinc sulfide (C.P.), antimony sulfide (C.P.); the following phosphors obtained from the E. I. du Pont 'de Nernours & Company-511 (zinc oxide), 601 (zinc silicate), 1200 (zinc-cadmium sulfide); the following phosphors from RCA-F-2032 (zinc oxide), F2039 (zinc-cadmium sulfide) and F- 2046 (zinc sulfide); and the following phosphors from New Jersey Zinc Company2ll0 (zinc-magnesium oxide), 2115 (zinc magnesium oxide), 2200 (zinc sulfide), 2205 (zinc sulfide), 2215 (zinc cadmium sulfide), 2301 (zinc sulfide), 2304 (zinc-cadmium sulfide), 2330 (zinc sulfide) and 2469 (calcium strontium sulfide). The chemically pure zinc sulfide and Phosphor 2330 were found useful only with negative sensitization. The antimony sulfide and Phosphors 511, 2215 and 2301 while useful with both polarities of sensitization were slightly more sensitive for positive sensitization. Phosphors 601, 1200, 2032, 2200, 2304 and 2469 while useful with both polarties of sensitization showed a preferential light sensitivity when negatively charged. The remaining materials were approximately equally sensitive for both polarities of charging.

Examples 1 9 Through 30 A series of xerographic plates were prepared by making a 1 to 1 mixture, by volume, of a particular photoconductive material with the silicone resin DC-996. This mixture was spread over a 4 x 5-inch aluminum plate by means of a doctor plate to give a coating approximately 0.007-inch thick. The photoconductive materials used in these examples were the following phosphors obtained from the New Jersey Zinc Company: 2100 (zinc oxide), 2110 (zinc magnesium oxide), 2115 (zinc-magnesium oxide), 2200 (zinc sulfide), 2205 (zinc sulfide), 2215 (zinc-cadmium sulfide), 2220 (zinc-cadmium sulfide), 2301 (zinc sulfide), 2304 (zinc-cadmium sulfide), 2469 (calcium-strontium sulfide), 2479 (calcium-strontium sulfide) and 2703 (calcium-strontium sulfide). In each case xerographic plates obtained with these ingredients were operable as in Example 1 to accept an electrostatic charge and to dissipate the charge upon illumination. The sulfur-containing photoconductors showed a higher light sensitivity than the oxygen-containing photoconductors. Accordingly, these xerographic plates of Examples 22 through 30, inclusive, were retested as described excepting that instead of uniform light they were exposed to light projected through a transparent photographic positive of a continuous-tone subject to produce on the plate a pattern of electrostatic charges corresponding to the pattern of light and shadow to be reproduced. The electrostatic charges were developed using the powder cloud development apparatus and method described in detail in US. 2,784,109 by Lewis E. Walkup. In each case an accurate reproduction of the continuous-tone original was obtained.

Examples 3] Through 33 A series of three xerographic plates were prepared as described in Examples 19 through 30 except that in Ex ample 31, 0.5 milliliter of a 1% toluene solution of monoaluminum stearate for each 5 grams of photoconductor were added to the resin-photoconductor mix. In Example 32 the same amount of dialluminum stearate and in Example 33 the same amount of trialuminum stearate was used. In each case the resin was silicone resin DC-996 and the photoconductor was Phosphor F-2039 (zinc-cadmium sulfide). It was found that the aluminum soaps tried were equally satisfactory and in each case produced a significant improvement in the texture of the plate surface and the quality of xerographic powder images developed on the surface as described in Examples 22 through 30.

Examples 34 and 35 Two xerog-raphic plates were prepared using a 1 to 1 mixture, by volume, of Phosphor 2225 (zinccadmium sulfide) and silicone resin DC-996. The mixture was thoroughly agitated to disperse the photoconduotor in the resin solution. In Example 34 the photoconduotorbinder mixture was coated on a 4 x 5-inch aluminum sheet using a doctor blade giving a coating of 0.005-inch thick. In Example 35, the mixture was sprayed on the aluminum plate giving a coating 0.003 inch thick. Both plates were then tested and were found to accept an electrostatic charge and to dissipate the charge upon illumination. There was no significant diiference between the plates in their xerographic properties or physical properties.

Examples 36 Through 39 A series of four xerographic plates were prepared using Phosphor 1 -2039 and silicone resin Bil-996. The photoconductor-binder mixture was applied to the 4 x 5 aluminum sheets by dipping the plates in the solution and withdrawing them at the rate of one-inch per minute. In Example 36 plate was Clipped once giving a coating thickness of 0.0035 inch. In Example 37 the plate was dipped twice giving a coating thickness of 0.007 inch. in Example 38 the plate was dipped three times giving a coating thickness of 0.011 inch and in Example 39 the plate was dipped four times giving a coating thickness of 0.0145 inch. The plates were then charged and charge acceptance and light and dark decay measured as in Examples 2 through 18. The results are set forth for both positive and negative charging in FIGS. 1 through 4.

It can be seen from the figures that the maximum poten ial accepted by the plate increases with the thickness of the photoconductive layer from 0.0035 to 0.007 inch. When the thickness increases beyond 0.007 inch, no significant change in accepted potential is observed. Continuous tone prints were also made on these plates as described for Examples 22-30. Improvement in the tone quality of the prints made on these plates followed the same pattern as charge acceptancetone improved up to a thickness of 0.007 inch with no further improvement observed with further increases in thickness.

Example 40 A l to mixture, by volume, of Phosphor F2039 and silicone resin DC-996 was prepared. The mixture was then ball-milled for four hours. At the end of this time particle size had been reduced from the range of 5 to 50 microns to a range of 0.5 to 5 microns. This treatment eliminated the fluorescence of the phosphor. A continuous tone xerographic image was developed on this plate as described in Examples 22 through 30. There was observed a substantial improvement in grain quality which was ascribed to the reduction in particle size of the photoconductor.

Examples 41 Through 43 A series of three xerographic plates were prepared us ing a 1 to 1, by volume, mixture of silicone DC-996 and, respectively, Phosphor 1200 (zinc-cadmium sulfide), Phosphor 1 -2039 (zinc-cadmium sulfide) and Phosphor 2225 (zinc-cadmium sulfide). The binder-photoconductor mixtures were applied to 4 x 5 aluminum. sheets using a doctor blade to give a coating about 0.005 inch thick. Phosphor 1200 had a particle size of about 75 l. l microns or more, Phosphor F2039 had a particle size of about to 30 microns and Ehosphor 2225 had a particle size or about 0.5 to 5 microns. The plates so prepared were electrically charged and charge acceptance and light and dark decay determined for both positive and negative electrical sensitization. The results are shown in FIGS. 5 and 6.

Example 44 Eighty parts of titanium dioxide, by weight and twenty parts, by weight, of polystyrene were mixed with sufficient toluene to give a free flowing solution. The suspension was fiowed onto a 4 x 5 aluminum sheet and air dried at room temperature. The plate was then baked for 2 /2 days at 65 C. and then at 120 C. for 5 hours. The plate so prepared was then tested for electrical charge acceptance and light and dark decay as in Examples 2-18. The plate accepted both positive and negative electrostatic charges and was appriximately equally light sensitive for both polarities of charging. The plate was approximately as sensitive to light as a plate coated with anthracene by vacuum evaporation but appeared to be rather humidity sensitive.

Example 45 A xerographic plate was prepared by adding to a porcelain ball-mill 2.1 parts by weight of a commercial antimony trisulfide obtained from the l. T. Baker Chemical Company, 1.0 part by weight of a poly-n-butyl methacrylate resin obtained from E. I. du Pont de Nemours & Company under the trade name Lucite 44 and 2.3 parts by weight of toluene. The mixture was ball-milled for 3 hours using porcelain balls about 0.5 inch in diameter. The mixture Was whirl-coated on a 4 x S-inch aluminum plate rotating at about 120 r.p.m. to give a coating about 61 microns thick. An electrostatic charge was placed on the plate using corona charging as described in US. 2,777,957 to L. E. Walkup. Initial charge acceptance, dark decay rate and light sensitivity were then determined using a vibrating probe electrometer. The light source used was a 60 watt incandescent lamp operating at a color temperature of 2775 K. to provide an illumination of 36 foot-candles on the plate. Under these conditions the plate was found to accept both positive and negative electrostatic charges and to show sensitivity to light for both polarities of sensitization. The plate had slightly better charge acceptance and light decay characteristics for negative sensitization than for positive.

Example 46 A xerographic plate was prepared and tested as in Example 45 using 3.68 parts of GP. grade HgS (l. T. Baker Chemical Co.) to 1.0 part Lucite 44 to 2.3 parts toluene (all parts by weight). The coating was 23 microns thick. The plate accepted both positive and negative electrostatic charges and was found to dissipate these charges upon illumination as in Example 45.

Example 47 A xerographic plate was prepared and tested as in Example 45 using 5.1 parts of GP. grade mercuric oxide (J. T. Baker Chemical Company) to 1.0 part of Lucite 44 to 1.3 parts of toluene (all parts by weight) and ballmilling for 8 hours. The coating was 25 microns thick. The plate was operable to accept an electrostatic charge and to dissipate the charge upon illumination using both positive and negative electrical sensitization.

Example 48 A xexrographic plate was prepared and tested as in Example 45 using 1.5 parts of chemically pure indium trisulfide, 1 part of a copolymer of n-butyl and isobutyl mcthacrylate obtained from the E. I. du Pont de Nemours and Company under the trade name of Lucite 46 and 4 parts of toluene (all parts by weight). The mixture was ball-milled for 6 hours using Pyrex glass beads T2 6 millimeters in diameter. The coating was 17 microns thick. The plate was operable to accept both positive and negative electrostatic charges and to dissipate the charges upon illumination as in Example 47.

Example 49 A xerographic plate was prepared and tested as in Example 48 using 1.5 parts of chemically pure AS 8 (Coleman and Bell Co.), 1.0 part of Lucite 46 and 3 parts of toluene (all parts by weight) and ball-milling for 5 hours. The coating was 53 microns thick. The xerographic plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges on illumination. The charge acceptance and light sensitivity of the plate was slightly better for positive sensitization than for negative.

Examples 50 Through 52 A series of three xerographic plates were prepared as in Example 45 using as the pigment Pb O Because of the high density of the pigment the ratio of binder to pigment is based on true volume using the density of the binder and pigment to compute the actual volume of the materials used. In each case 1 part by volume of Lucite 46 was used as the binder. The amounts of Pb O for Examples 50-52 Were, by volume, 0.55, 0.88 and 1.0, respectively, to 5.8, 6.8 and 7.4 volumes of toluene, respectively. The coatings were about 22 microns thick. In each case the xerographic plates were operable to accept either positive or negative charges and to dissipate the charges upon illumination. Upon retesting, using a monochromatic lighting source, it was found that the peak sensitivity of these plates was at about 500 millimicrons Wavelength of the incident light. Spectral sensitivity in this and in the succeeding examples was determined using a Backmau spectrophotometer at an intensity of 0.12 microwatt per square centimeter.

Example 53 A xerographic plate was prepared and tested as in Example 45 using 1.4 parts of phosphor grade CdSe (copper activated, Merck & Co.), 1 part Lucite 44 and 5 parts of toluene and the solution ball-milled for 19 hours. Additional toluene and Lucite 44 were added to change the volume ratio to 0.7 part cadmium selenide to 1 part of Lucite 44 (all parts by volume, adjusted by density to give true volume). The mixture was ball-milled an additional hour and a 4 x 5-inch aluminum plate whirlcoated to give a coating 25 microns thick. The plate so obtained was operable to accept an electrostatic charge of either polarity and to dissipate the charge upon illumination. On retesting with a monochromatic light source, as in Examples 50 through 52, it was found that the plate showed photosensitivity over the wavelength range of 650 to 800 millimicrons. The peak sensitivity was at 750 millimicrons.

Example 54 A xerographic plate was prepared and tested as in Example 45 using 1.56 parts of a chemically pure arsenic trisulfide, 1 part of Lucite 46 and 5.6 parts of toluene (all parts by weight) and ball-milling for 6.5 hours. The coating was 20 microns thick. The plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges upon illumination to light.

Example 55 A xerographic plate was prepared and tested as in Example 45 using 1.34 parts of CF. grade gallium triselenide, 1 part of Lucite 46, 5.66 parts of toluene (all parts by weight) and ball-milling for 6.5 hours. The coating was 18 microns thick. The xerographic plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges upon illumination. The plate was sensitive over the entire range of wavelengths from 400 to 700 millimicrons with peak sensitivity at about 600. The plate showed a higher light sensitivity when positively sensitized.

Example 56 A xerographic plate was prepared and tested as in Example 45 using 2.5 parts of Phosphor 2205 (zinccadmium sulfide), 1 part of Lucite 46 and 5.66 parts by weight of toluene (all parts by weight), and ball-milling for 7.5 hours. The coating was microns thick. The plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges on illumination. The plate had slightly better xerograpln'c properties for negative sensitization than for positive.

Example 57 A xerographic plate was prepared and tested as in Example 45 using 5.0 parts of Phosphor 2330 (zinccadmium sulfide), 1 part of Lucite 46 and 5.66 parts of toluene (all parts by weight), and ball-milling for 7.5 hours. The coating was 41 microns thick. The plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges on illumination. The plate was sensitive only in the bluegreen with peak sensitivity in the near ultraviolet.

Example 58 A xerographic plate was prepared and tested as in Example 45 using 5.0 parts of Phosphor 2225 (zinccadmium sulfide), 1 part of Lucite 46, and 5.66 parts of toluene (all parts by weight), and ball-milling for 17 hours. The coating was 28 microns thick. The plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges on illumination. The plate had slightly better charge acceptance and light sensitivity when negatively sensitized than when positively sensitized. Light sensitivity extended from the near ultraviolet out to the orange (about a wavelength of 600 millimicrons). Peak sensitivity was at about 500 millimicrons.

Example 59 A xerographic plate was prepared and tested as in Example 45 using 2.5 parts of Phosphor F4039 (zinccadmium sulfide), 1 part of Lucite 46 and 3.0 parts of toluene (all parts by weight), and ball-milling 17.5 hours. The coating was 46 microns thick. The plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges on illumination. The plate had much more light sensitivity for positive sensitization than for negative. The plate was sensitive over the entire visible spectrum with a peak at a wavelength of about 500 millimicrons when negatively charged.

Example 60 A xerographic plate was prepared and tested as in Example 45 using 2.5 parts of Phosphor 1200 (zinccadmium sulfide), 1 part of Lucite 46 and 3.0 parts of toluene (all parts by weight), and ball-milling 17 hours. The coating was 43 microns thick. The plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges on illumination.

Example 61 A xerographic plate was prepared and tested as in Example 45 using 2.5 parts of Phosphor 2703 (calciumstrontium sulfide), 1 part of Lucite 46 and 5.66 parts of toluene (all parts by Weight), and ball-milling for 7 hours. The coating was 23 microns thick. The plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges on illumination.

Example 62 Example 63 A xerographic plate was prepared and tested as in Example using 1.8 parts of a GP. grade Zinc sulfide, 1.0 part of a silicone resin obtained from General Electric and sold under the trade name SR82, 3.2 parts of toluene (all parts by weight), and ball-milling for 21 hours. The coating was 106 microns thick. The plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges on illumination. This plate displayed much better charge acceptance and light sensitivity when negatively charged. Moreover, it Was particularly outstanding in having extremely low residual potentials after light exposure.

Example 64 A xerographic plate was prepared and tested as in Example 45 using 2.5 parts of a GP. grade Pb o 1 part of Lucite 46, 4 parts of toluene (all parts by weight), and ball-milling for 5 hours. The coating was 23 microns thick. The plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges on illumination.

Example 65 A xerographic plate was prepared and tested as in Example 45 using 0.44 part of a GP. grade cadmium sulfide, 1 part of Lucite 46, 4 parts of toluene (all parts by weight) and ball-milling for 3 hours. The coating was 20 microns thick. The plate so prepared was operable to accept both positive and negative electrostatic charges and to dissipate the charges on illumination. This plate displayed a definitely higher charge acceptance and light sensitivity for negative charge than for positive.

Example 66 A xerographic plate was prepared and tested as Example 45 using 5 parts of Phosphor 2225 (zinc-cadmium sulfide), 1 part of CE. grade mercuric sulfide, 1 part of Lucite 46 and 5 parts of toluene (all parts by true volume). The ratio of total pigment to binder was 1.4 to 1 by volume. The mixture was ball-milled for 15 hours and whirl-coated on an aluminum plate to give a coating 28 microns thick. The plate so prepared accepted relatively high electrical potentials of either positive or ne ative polarity with very low dark decay rates. The plate had a low but definite sensitivity over the entire spectral range from 375 to 700 millimicrous with peak sensitivity at about 600. As compared to the light sensitivities of the separate pigments, peak sensitivity was less for this plate and extended much further into the red.

Example 67 A xerographic plate was prepared and tested as in Example 45 using 10 parts zinc oxide (a pigment grade obtained from New lersey Zinc Co. under the trade name Florence Green Seal No. 8), 1 part C.P. grade arsenic triselenide, 1 part Lucite 46, 6.3 parts of toluene (all parts by true volume) and ball-milling for 15 hours to give a coating microns thick. The volume ratio of total pigment to binder was 1.4 to 1. As noted in Example 62, arsenic triselenide has a peak sensitivity in the red. Zinc oxide has a peak sensitivity in the near ultraviolet. This plate with the pigment mixture showed a high photosensitivity at 375 millirnicrons (substantially higher than Zinc oxide alone) falling off to virtually zero at 400 millimicrons. No photo-sensitivity could be detected at longer wavelengths. The separate sensitivity of the arsenic triselenide was apparently quenched. The plate did not accept a positive charge.

Examples 68 Through 69 A series of three xerographic plates were prepared and tested as in Example 45 using parts of Florence Green Seal No. 8 zinc oxide, 1 part of a phosphor, one part of Lucite 4-6, 5.5 parts of toluene and ball-milling for 14 hours. The phosphors for Examples 68 through 69 were 2225, 2330 and F2Q39, with coating thicknesses of 28 microns, 13 microns and 23 microns, respectively. The ratio of pigment to binder was 1: 1. All parts are by true volume. The spectral sensitivity of the plates were determined and the results are shown in FIGURE 7.

Example 70 A xerographic plate was prepared by using 2.5 parts by weight of Florence Green Seal No. 8 zinc oxide, 1 part by weight SR-82 and sufficient toluene to give good grinding viscosity and ball-milled to uniformly disperse the pigment in the binder. The resulting mixture was whirlcoated on an aluminum plate and air dried for 48 hours. The coating was 23 microns thick. The spectral sensitivity of the plate was determined and the results are shown in FIG. 7.

Relative white-light sensitivities of various plates were calculated for sunlight and photofiood light by numerical integration of the emission curve of the light source and the spectral sensitivity curve of the photoactive material. The curve for sunlight was average from data given by Abbott, Progress Committee Report, Journal of the Optical Society of America, 10, 234 (1925), and by Bulletin LD1, published by the Nela Park Engineering Division, General Electric Co., 1946. The curves used for these light sources are shown in FIG.

The spectral sensitivities of the photoactive materials were calculated using the equation a 1 I T T where:

T is the time in seconds for the potential on the plate to decay in the dark to one-half of some given value,

T is the time in seconds for the potential on the plate to decay under given illumination to one-half of the same initial value used in the determination of T and I is the intensity of the light in microwatts per square centimeter.

The spectral sensitivities to sunlight of the plates of Examples 46, 48, 55, 59, 62 and 70 were then determined. The spectral sensitivity to sunlight was similariiy calculated for a commercial xerographic plate comprising vacuum evaporated selenium on an aluminum backing obtained from The Haloid Company, Rochester, New York. The results are shown in bar-graph form in FIG. 8 with the numbers normalized to 100 for selenium. The same calculation was then repeated to determine the relative spectral sensitivity of these plates to a photofiood source. In addition, the spectral sensitivity of the plate in Example 68 was calculated for this light source. The relative sensitivity in the terms of 100 for the commercial selenium plate is shown in bar-graph form in FIG. 9.

The choice of a particular pigment will depend upon the needs of the formula-tor. Thus, if spectral sensitivity is the determining factor, pigment grade zinc oxide is sensitive only in the far blue and near ultraviolet while cadmium selenide is sensitive only in the red and near infrared. Mercuric sulfide is sensitive only in the range Within about 25 millimicrons of 600. Arsenic triselenide is sensitive over the entire visible spectrum with a peak in the far red. Gallium trisclcnide and indium trisulfide are sensitive from the green to far red with a high peak in the orange. Costs vary widely. Pigment grade zinc oxide is relatively cheap but has low sensitivity. Gallium triselenide, indium trisulfide and arsenic triselenide, although quite sensitive to incandescent light sources are relatively expansive and not readily available commercially.

A particularly preferred material for use in xerographic binder plates are the zinc-cadmium sulfide phosphors. These materials are readily available commercially and in themselves offer an extremely wide range of choice in spectral characteristics. Thus, Phosphor 23 30 is sensitive only in the near ultraviolet to green region of the spectrum while Phosphor F2039 is sensitive over the entire spectral range from the near ultraviolet to the far red. Whereas materials such as pigment grade zinc oxide and GP. grade zinc sulfide are generally useful xerographically only when neagtively charged, the zinc-cadmium sulfide phosphors are usable when sensitized with either polarity of electrical charge. These materials also offer a wide variety of choice in modifying their properties as by combining with other pigments as in the case of combining with the cheaper zinc oxide pigment to give sensitivity to the green (in the case of Phosphor 2225) combined with an extremely high peak in the near ultraviolet. This extremely high peak sensitivity makes such a combination eminently suit-able for dye sensitization (as with rose bengal or other known photographic sensitizing dyes) which would further extend the sensitivity at longer wavelengths. These and other factors will, of course, be obvious to those skilled in the art.

As can be seen in the previous examples, the thickness of the photoconductive insulating layer is not critical. In general, the layer may be anywhere from about 10 to 200 microns thick. For best operation it is preferred that the layer not be over about microns thick.

In general, the ratio between binder and the inorganic photoconductive insulating compound is from about 1 part binder and 10 parts photoconductor to about 2 parts binder and 1 part photoconductor by weight. The actual proportion will, of course, depend on the specific binder as well as on the properties and characteristics desired. As a general guide, it is indicated that the quantities of the binder should be the least amount which will adequately secure the photoconductor to the surface of the backing member and which will form a smooth and useful surface for the ultimate deposition thereon of electr-ostatically charged powder particles.

The mechanism of operation of a xerographic binder plate is not understood. It has been postulated that the individual particles of operable inorganic compounds exist with two different conductive states in the particles, that is, the surface may be p-type while the interior of the particles may be n-type or vice versa. The difference of conductivity type may be due to the method of the preparation of the crystal or to the interaction of the binder in the preparation of the xerographic plate. These pigment particles may form irregular chains of conductive paths leading from the surface of the binder plate to the interface with the backing material. Such a path would offer a large number of p-n junctions.

Alternatively, the pigment particles may be coated with a thin insulating film of binder offering a resistive barrier between adjacent pigment particles. As compared to amorphous photoconductive films wherein carrier movement is controlled primarily by trapping and consequent space charges, and perhaps secondarily by interchain potential barriers, in the binder plate, carrier movement is probably limited primarily by interparticle barriers. It is not intended to limit the invention to this or any other theory of operation. The critical requirement or property, as discussed hcreinbefore, is that the excited photoelectrons or holes left behind can migrate when under an applied electric field. Regardless of theory of operation, it is observed that the xerographic members a-- cording to this invention have a very high specific resistivity in the absence of activating illumination and are such as to have a decay factor as previously defined of less than about 30, generally less than about 0.1. Typical compounds possessing these properties are cadmium sulfide, zinc sulfide, cadmium selenide, zinc selenide, mixed sulfides or selenides of these metals and other compounds sometimes available under the class of phosphors and believed to have activating crystal imperfections or impurities, such as amounts of other elements, for example, up to about 1% and generally about 0.001 to 0.01% of elements, such as copper, zinc, calcium, silver, magnesium and the like.

This application is a continuation-in-part of our earlier filed co-pending application Ser. No. 311,546, filed on September 25, 1952, which in turn was a continuation in-part of Ser. No. 95,374, filed May 25, 1949, by Arthur E. Middleton now US. Patent 2,663,636.

What is claimed is:

1. A process for recording a pattern of light and shadow comprising in the absence of activating radiation placing sensitizing electrostatic charges of one polarity on the surface of a xcrographically sensitive member comprising a conductive backing and a thin photoconductive insulating layer thereon comprising an insulating organic resin binder and dispersed therein finely-divided particles of an inorganic photoconductive insulating metallicdons containing crystalline compound having electrons in the nonconductive energy level activatable by illumination to a different energy level whereby an electric charge is free to migrate under an applied electric field in the order of at least volts per cm., the composite resistivity of the layer being at least 10 ohms-cm. in the absence of illumination and having a decay factor of less than 3.0, exposing the thus charged surface to a pattern of light and shadow to be recorded whereby an electrostatic latent image is formed corresponding to said pattern and de positing electrically attractable finely-divided marking material selectively in conformity with the electrostatic image thus produced.

2. A process according to claim 1 wherein said compound is zinc sulfide.

3. A process according to claim pound is cadmium sulfide.

4. A process according to claim pound is zinc selenide.

5. A process according to claim pound is cadmium selenide.

6. A process according to claim pound is titanium dioxide.

7. A process for recording a pattern of light and shadow comprising in the absence of activating radiation placing sensitizing electrostatic charges of one polarity on the surface of a xerographically sensitive member comprising a conductive backing and a thin photoconductive insulating layer thereon comprising an insulating organic resin binder and dispersed therein finely-divided particles of an inorganic photoconductive insulating metallic-ion containing crystalline phosphor, said phosphor showing photoluminescence when excited by low energy photons, exposing the thus charged surface to a pattern of light and shadow to be recorded whereby an electrostatic latent image is formed corresponding to said pattern and depositing electrically attractable finely-divided marking material selectively in conformity with the electrostatic image thus produced.

8. A process according to claim 7 wherein said particles comprise a phosphor-grade zinc chalkogenide, the chalkogen having an atomic weight not more than about 128.

1 wherein said com- 1 wherein said com- 1 wherein said com- 1 wherein said com- 9. A process according to claim 7 wherein said particles comprise a phosphor-grade zinc sulfide.

10. A process according to claim 7 wherein said particles comprise a phosphor-grade zinc selenide..

11. A process according to claim 7 wherein said particles comprise a phosphor-grade cadmium chalkogenide, the chalkogen having an atomic weight not more than about 128.

12. A process according to claim 7 wherein said particles comprise a phosphor-grade cadmium sulfide.

13. A process according to claim 7 wherein said particles comprise a phosphor-grade cadmium selenide.

14. A xerographic process comprising imposing an electrostatic field through a photoconductive insulating layer of an insulating organic resin binder having dispersed therein finely divided particles of an inorganic photoconductive insulating metallic-ion containing crystalline compound having electrons in the non-conductive energy level activatable by illumination to a different energy level whereby an electric charge is free to migrate under an applied electric field in the order of at least 10 volts per cm., the composite resistivity of the layer being at least 10 ohms-cm. in the absence of illumination and having a decay factor of less than 3.0, said photoconductive insulating layer being positioned in electrical contact with a non-light sensitive electrically conductive backing and, while the field is imposed, selectively flowing charge through portions of the photoconductive insulating layer by selectively exposing said portions to activating radiation forming a varying charge pattern of intelligence to be reproduced which is adapted to be developed with marking material.

15. A xerographio process comprising imposing an electrostatic field through a photoconductive insulating layer of an insulating organic resin binder having dispersed therein finely divided particles of an inorganic photoconductive insulating metallic-ion containing crystalline phosphor, said phosphor showing photoluminescence when excited by low energy photons, said photoconductive insulating layer being positioned in electrical contact with a nonlight sensitive electrically conductive backing and, while the field is imposed, selectively flowing charge through portions of the photoconductive insulating layer by selectively exposing said portions to activating radiation forming a varying charge pattern of intelligence to be reproduced which is adapted to be developed with marking material.

References Cited in the file of this patent UNITED STATES PATENTS 2,169,840 Lewis et a1 Aug. 15, 1939 2,297,691 Carlson Oct. 6, 1942 2,408,475 Nickle Oct. 1, 1946 2,599,542 Carlson June 10, 1952 2,663,636 Middleton Dec. 22, 1953 2,692,178 Grandadam Oct. 19, 1954 2,735,785 Greig Feb. 21, 1956 2,811,465 Greig Oct. 29, 1957 2,857,271 Sugarman Oct. 21, 1958 2,901,348 Dessauer et al. Aug. 25, 1959 FOREIGN PATENTS 201,301 Australia Apr. 21, 1955 OTHER REFERENCES McNaughton: Paint Manufacture, March 1937, pages 82 and 84.

Wainer: Photographic Engineering, vol. 3, No. 1, 1952, pages 12 to 22, originally presented May 24, 1951.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 121,006 Februaryll, 1964 Arthur E, Middleton et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 57, for "mteal" read metal column 2, line 66, for "insualting" read insulating column 3, line 73, for "requsite" read M requisite column 4, line 72, for "increased" read increases column 5, line 7, for "unsuspeceted" read unsuspected column 6, line 14, for "mixture" read mixtures column 9, line 31, for "po'larties" read polarities column 11, line 18, for "appriximately" read approximately line 69, for "xexrographic" read xerographic column 15, line 3:5, for"average" read averaged column 16, line 6, for "expansive" read expensive line 18, for "neagtively" read negatively column 17, line 26, for "metallic-ions" read metallic-ion Signed and sealed this 11th day of August 19 4.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A PROCESS FOR RECORDING A PATTERN OF LIGHT AND SHADOW COMPRISING IN THE ABSENCE OF ACTIVATING RADIATION PLACING SENSITIZING ELECTROSTATIC CHARGBES OF ONE POLARITY ON THE SURFACE OF A XEROGRAPHICALLY SENSITIVE MEMBER COMPRISING A CONDUCTIVE BACKING AND A THIN PHOTOCONDUCTIVE INSULATING LAYER THEREON COMPRISING AN INSULATING ORGANIC RESIN BINDER AND DISPERSED THEREIN FINELY-DIVIDED PARTICLES OF AN INORGANIC PHOTOCONDUCTIVE INSULATING METALLIC-IONS CONTAINING CRYSTALLINE COMPOUND HAVING ELECTRONS IN THE NONCONDUCTIVE ENERGY ELVEL ACTIVATABLE BY ILLUMINATION TO A DIFFERENT ENERGY ELVEL WHEREBY AN ELECTRIC CHARGE IS FREE TO MIGRATE UNDER AN APPLIED ELECTRIC FIELD IN THE ORDER OF AT LEAST 10**3 VOLTS PER CM., THE COMPOSITE RESISTIVITY OF THE LAYER BEING AT LEAST 10**10 OHMS-CM. IN THE ABSENCE F ILLUMINATION AND HAVING A DECAY FACTOR OF LESS THAN 3.0, EXPOSING THE THUS CHARGED SURFACE TO A PATTERN OF LIGHT AND SHADOW TO BE RECORDED WHEREBY AN ELECTROSTATIC LATENT IMAGE IS FORMED CORRESPONDING TO SAID PATTERNA ND DEPOSITING ELECTRICALLY ATTRACTABLE FINELY-DIVIDED MARKING MATERIAL SELECTIVELY IN CONFORMITY WITH THE ELECTROSTATIC IMAGE THUS PRODUCED. 